The macroscopic and microscopic anatomy of the developing human lung will be studied in human fetal specimens. Airway, vascular, and lymphatic development will be evaluated in staged human fetuses from thirty days gestation to term. This thorough analysis of the normal anatomy will serve as a foundation for anatomical and radiographic studies of selected specimens with lung anomalies and malformations. Macroradiographic, microradiographic, stereomicroradiographic, and histologic methods will be applied and perfected in the fetal dog lung. Then these techniques, which allow the spatial relationships within the lung to be directly correlated with histology, will be systematically applied to the large staged human fetal collection of the Human Growth and Development Study of Yale University. This data on normal fetuses will be correlated with existing data from classical serially sectioned embryos. Selected abnormal human fetal specimens will be similarly examined.